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Prevalence of asymmetrical rupture in small earthquakes and its effect on the estimation of stress drop: a systematic investigation in inland Japan
Geoscience Letters ( IF 4 ) Pub Date : 2019-12-12 , DOI: 10.1186/s40562-019-0145-z
Keisuke Yoshida

Stress drops of small earthquakes have been estimated under the assumption that the rupture propagates symmetrically within a circular fault. However, recent studies have observed directivity effects on seismic waves even for small earthquakes. In this study, rupture directivity was investigated systematically for small-to-moderate-sized earthquakes (M 3.5–5.5) that occurred beneath inland Japan from 2004 to April 2019. Apparent moment rate functions were determined for 1463 earthquakes, and their corner frequencies were inverted for their rupture parameters. The results indicate that most of the analyzed earthquakes (1217 of 1463) are characterized by significantly asymmetrical rupture propagation. The stress drops of the earthquakes estimated by considering asymmetrical rupture propagation were 16.8 MPa, which are almost twice the estimates based on the commonly used symmetrical rupture model. This shows the importance of recognizing the diversity of ruptures, even for small earthquakes, for extracting information about earthquake sources and the Earth’s structure. The prevailing rupture directivity can provide useful information on source parameters including the fault size, fault geometry, and rupture velocity of small earthquakes.

中文翻译:

小地震中不对称破裂的普遍性及其对应力降估算的影响:日本内陆的系统调查

在假定破裂在圆形断层内对称传播的前提下,估计了小地震的应力降。然而,最近的研究已经观察到方向性对地震波的影响,即使对于小地震也是如此。在这项研究中,系统地研究了2004年至2019年4月在日本内陆以下发生的中小地震(M 3.5-5.5)的破裂指向性。确定了1463次地震的表观矩速率函数,其转折频率为倒置其破裂参数。结果表明,大多数已分析的地震(1463年的1217年)的特征都是破裂明显不对称。考虑不对称破裂传播估计的地震应力降为16.8 MPa,这几乎是根据常用对称破裂模型估算的两倍。这表明,即使对于小地震,也要认识到破裂的多样性,以提取有关地震震源和地球结构的信息的重要性。普遍的破裂方向性可以提供有关震源参数的有用信息,包括断层大小,断层几何形状和小地震的破裂速度。
更新日期:2019-12-12
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